Plating-bath



UNITED STATES PATENT OFFICE.

MESNE ASSIGNMENTS, TO EASTERN MANUFACTURING COMPANY, OF. BOSTON, 'MASSAGHUSETTS, A. CORPORATION OF MASSACHUSETTS.

I PLATIENG-IBATH.

1T0 Drawing.

have invented new and useful Improvements in Plating-Baths, of which the following is a specification.

In the electrical deposition of zinc from a solution which consists solely of an electrolyte, there is a tendency for the zinc to deposit .in a granular lusterless gray layer,

which may, if'the current density is high, develop marked surface irregularities. n the art, as now practised, it is well known that this tendency to irregularities is reduced by the addition of a colloidal mate rial, such as glucose or glue.

We have discovered that a much better result than that found in current practice may be achieved by the addition of waste sulfite liquor, or of the residue obtained by evaporating the sulfite liquor to dryness. This waste sulfite liquor is the material discharged from digesters which have been used for cooking wood chips in the production of sulfite pulp for the manufacture of paper. The chemical composition of" this liquor, and of the solids contained in solution therein, is highly complex. In general it consists of a solution of lignin, ligno cellulose and resinous materials dissolved out of the chips by the action of sulfurous acid. in combination with lime and magnesia, used as the cooking acid in the sulfite process.

We have found an entirely satisfactory result may be obtained by adding to the plating bath about .015% of the dry residue obtained by evaporating sulfite liquor. A suitable plating bath consists of a solution containing 10 to 12% zinc sulfate, 15% magnesium sulfate and .1'% free sulfuric acid, but of course this bath may be varied as. desired. We have found that an entirely satisfactory result may be obtained by adding to the plating bath about .015'% of the dry residue obtained by evaporating waste sulfite liquor. No detrimental results acerue from the use of the higher percentage of sulfite liquor residue up to ten times the amount specified. Probablystill higher concentrations would do no more than produce foam. On the other hand, the percentage Specification of Letters Patent.

Patented May 31,- 1921.

. Application filed November 17, 1919. Serial No. 338,734.

specified gives results as satisfactory in all respects as those obtained with higher percentages.

As a means of rendering assistance to further investigation, our theory of the reason for this result is hereinafter outlined, but the facts are as stated above, whether or not our theory may prove to be correct;

and the improvement in plating which we have achieved can be reliably duplicated by any person skilled in the art who merely follows the directions already given.

It is more or less generally held that colloids present in the electrolyte congregate rapidly at any point on the cathode where there is excessive local current-density, and conversely are less numerous at points of local deficiency in current density; and we believe that this action is materially assisted in the case of our addition agent, by its incipient electrolytic properties.

Whether or not this theory points the way to the true cause, the effect is to produce, even at relatively high current-densities, a continuous and coherent electrolytic zinc plating having almost the specular surface of hot-dipped tin-plate, a valuable result which has heretofore been sought without success.

Having thus described our, invention, what we claim as new and desire to secure by Letters Patent is:

1. An electro-plating bath for metals, which contains, in addition to a salt of the metal to be deposited, a relatively small proportion of the soluble material contained in the sulfite liquor resulting from cooking wood by the sulfite process.

2. An electro-plating bath for zinc, which contains, in addition to a zinc salt, a relatively small proportion of the soluble substance contained in the sulfite liquor resulting from cooking wood by the sulfite process.

3. An electro-plating bath for metals, which contains, in additon to a salt of the metal to be deposited, a relatively small proportion of the dried soluble residue obtained by evaporating waste sulfite liquor.

In testimony whereof We have afllxed our signatures. 

